Sprue bushing with cast in electrical heating element

ABSTRACT

This invention relates to an improved method of making a sprue bushing for injection molding and the improved sprue bushing made thereby. The runner passage extends through a corrosion resistant inner core portion with an outer surface around which a helical electrical heating element is located. A stainless steel outer sleeve extends between an enlarged front portion which provides the opening to the gate and an enlarged back collar portion which provides for connection to the heating element leads. The space around the heating element is filled by a highly conductive material such as a beryllium copper alloy in a vacuum furnace which causes the beryllium copper to fuse with the adjacent materials. This improves the transfer of heat from the heating element to the melt and the outer sleeve substantially reduces the costly machining required to produce the finished product as well as provides additional strength. In one embodiment, wells extending into the front portion are filled with the highly conductive material which increases heat transferred to the gate area. A thermocouple may be located in one of the wells to monitor the transfer of heat to this area.

This is a divisional of application Ser. No. 234,641, filed Feb. 17,1981, now U.S. Pat. No. 4,355,460, which is a continuation-in-part ofapplication Ser. No. 217,115 filed Dec. 17, 1980, now abandoned.

BACKGROUND OF THE INVENTION

This invention relates to injection molding and more particularly to animproved electrically heated sprue bushing and method of making thesame.

The sprue bushing of the present invention represents an improvementover the sprue bushing disclosed in the applicant's previous U.S. patentapplication Ser. No. 36,880 filed May 8, 1979. It relates to the sametype of structure with a helical heating coil embedded in a highly heatconductive material cast over an inner core portion formed of acorrosive resistant material, but it also includes a finished outersleeve portion. The previous sprue bushing has the disadvantage that itsouter surface is formed by the cast material. This requires that theouter surface of each sprue bushing be machined to provide the necessarysmooth finish. Furthermore, the highly conductive material which isusually a beryllium copper alloy does not have sufficient corrosionresistance to provide a durable outer casing of a sprue bushing. Thishas resulted in it requiring a costly nickel plating process to protectit from corrosive gases escaping from the gate area.

The process of making the applicant's previous sprue bushing thereforehas the serious disadvantages that costly machining and plating stepsare required after casting. Furthermore, the even application of heatalong the runner passage is critical and therefore it is important thatthere be optimum heat transfer between the heating element and thesurrounding conductive material and between the conductive material andthe inner core portion through which the runner passage extends.Resistance to heat transfer at the interfaces between the differentmaterials results in uneven temperature distribution which may shortenthe life of the heating element and may cause the melt to deteriorate.

SUMMARY OF THE INVENTION

Accordingly, it is an object to at least partially overcome thesedisadvantages by providing an improved method of making a sprue bushingand an improved sprue bushing produced thereby whereby the highlyconductive material is cast in a vacuum furnace over the heating elementbetween a corrosion resistant outer sleeve and a corrosion resistantinner core portion. This substantially reduces the finishing steps andimproves the bond of the conductive material with the adjacentmaterials.

To this end, in one of its aspects, the invention provides a method ofmanufacturing an integral electrically heated sprue bushing having ahollow elongated inner core portion defining a central runner passageextending between an enlarged front portion and an enlarged back collarportion, a helical heating element encircling the inner core portion, ahighly conductive elongated portion around the heating element, and anelongated outer sleeve portion over the conductive portion, comprisingthe steps of: (a) manually assembling the heating element onto the coreportion to extend between the front and back portions; (b) securing anouter sleeve in a position to enclose a space around the heating elementbetween the front and back portions to form an assembly with the spacebeing sufficiently sealed to prevent substantial leakage; (c) vacuumfilling the space with a highly conductive molten material to providethe intermediate portion; and (d) allowing the highly conductivematerial to cool sufficiently to solidify.

In another of its aspects, the invention further provides an integralelectrically heated sprue bushing comprising: a hollow elongated innercore portion defining a central runner passage extending therethrough,the inner core portion extending between an enlarged front portion andan enlarged back collar portion, the core portion being formed of acorrosion resistant material having an outer surface; a helical heatingelement having a plurality of coils encircling the inner core portionand lead wires extending through an aperture in the back collar portion,the inner helical diameter of the coils being greater than the maximumouter diameter of the outer surface of the inner core portion; anelongated outer sleeve portion extending between the front portion andthe back portion to enclose a first space around the heating elementbetween the front portion and the back portion; a high conductiveportion cast into said space between the inner core portion and theouter sleeve portion; and a further sleeve portion with one end receivedin the aperture in the back collar portion to receive the lead wiresextending therethrough, said further sleeve portion also being filledwith highly conductive material to encase the portion of the lead wiresextending therethrough.

Further objects and advantages of the invention will appear from thefollowing description taken together with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an isometric view of a sprue bushing showing partial assemblyaccording to a first embodiment of the invention;

FIG. 2 is a similar view illustrating an assembly ready for fillingaccording to the first embodiment;

FIG. 3 shows a number of assemblies which are placed in a vacuum furnacefor filling;

FIG. 4 is a sectional view showing a sprue bushing finished according tothe first embodiment of the invention;

FIGS. 5 and 6 are isometric views showing assembly for filling in thereverse direction according to a second embodiment of the invention;

FIG. 7 is a sectional view showing an assembly ready for filling ofanother embodiment of the sprue bushing according to a furtherembodiment of the invention;

FIG. 8 is a sectional view showing an assembly ready for filling of afurther embodiment of the sprue bushing according to the invention; and

FIG. 9 is a sectional view showing a sprue bushing according to yetanother embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference is first made to FIGS. 1 and 2 which show assembly of oneembodiment of the sprue bushing according to the preferred method. Theassembly 10 has a hollow elongated inner core portion 12 which, in thisembodiment, is integral with an enlarged front portion 14. A runnerpassage 16 extends centrally through the inner core portion and leads tochannels 18 which extend radially outward in the front face of the frontportion. In use, each of these radial channels conveys melt to acorresponding edge gate. As may be seen, the enlarged front portion 14has a number of holes or wells 20 extending therethrough, one betweeneach of the radial channels 18. The inner core portion 12 has anundulating outer surface 22 which forms a helical ridge 24 having auniform curved cross section.

The first step of assembly is to manually insert the inner core portionthrough a helical heating element 26 which has electrical leads 28 whichform an outwardly extending stem 30 at one end. The coils of the heatingelement are separated so that there is no contact between them and thehelical diameter of the heating element 26 is just slightly larger thanthe maximum outer diameter of the inner core portion 12. The helicalheating element 26 spirals in one direction while the helical ridge 24spirals in the opposite direction so that any contact between them isonly at the points where they intersect. The electrical leads or coldterminals 28 extend through the stem 30 to connect to an electricalsource (not shown) in a conventional manner.

The next step is to manually add an enlarged back collar portion 32which has a cylindrical seat 34 to receive the butt end 36 of the innercore portion 12 with a press fit. The back collar portion 32 has acircular aperture 38 through which the electrical leads 28 and stems 30must be inserted as the back collar portion 32 is added. Next, a largercylindrical sleeve 40 is installed over the inner core portion 12 and asmaller sleeve or enclosure 42 is installed over the stem. The outersleeve 40 is inserted into the back collar portion 32 to abut with apress fit on shoulder 44 and is just large enough to form a press fitwith the outer surface 46 of the front portion 14. As may best be seenin FIG. 2, the outer sleeve 40 is long enough to extend past the frontportion 14 and form an upwardly open mouth 48. The smaller sleeve 42 isinserted into the aperture 38 in the back collar portion 32 to abut onshoulder 50 and is retained in a press fit. It has an end portion orcuff 52 which defines a hole 54 through which the stem 30 extends with atight fit. Thus, the press fits of the various components are sufficientto provide an assembly 10 which is sufficiently stable to be selfsupporting. It has an enclosed space 56 around the heating element 26between the inner core portion 12 and the outer sleeve 40 which extendsinto a further space 58 around the stem 30.

The next step is to seal the joints between the various componentsagainst leakage. This includes the joints between the back collarportion 32 and both sleeves, and the joint between the smaller sleeve 42and the stem 30. In a preferred embodiment of the invention, this isdone by applying a small amount of brazing paste to each joint and thenheating the assembly in a vacuum furnace 60 to cause the paste to meltand run all around the joint to braze it and seal it against leakage.The temperature and length of time which the assembly is heated must besufficient to braze the components together and may also be sufficientto heat treat the components to eliminate a separate heat treating step.Two alternative sealing methods are to weld the joints or apply ceramiccement to them.

After sealing, a plug plate 62 is positioned on the front face of thefront portion 14 to cover the channels 18. It has holes 64 therethroughwhich are aligned with wells 20 and a slug 66 of highly conductivematerial is inserted into the mouth 48 formed by the projecting portionof the outer sleeve 42. The assembly is then inserted into the vacuumfurnace 60 in the upright position shown. It is heated until the slug 66is melted and the molten material flows downward through wells 20 tofill space 56 as well as the further space 58 in the smaller sleeve 42and the wells themselves. The degree and duration of this second heatingstep must be sufficient to melt the material and fill the spaces, butcannot be sufficient to release the previously brazed joints. In thepreferred embodiment, the inner core portion 12, the front portion 14and the back collar portion 32 are formed of a highly corrosionresistant material such as stainless steel to withstand the corrosiveeffects of the melt. The outer sleeve 40 and smaller sleeve 42 are alsostainless steel to provide a durable finish and to protect them againstcorrosive gases escaping from the gate area. The highly conductivematerial is a beryllium copper alloy, although other copper alloys maybe used in other embodiments. Filling under a partial vacuum results inthe beryllium copper fusing with the stainless steel and the heatingelement 26, improving the bond therebetween, and thus increasing theheat transfer from the heating element 26 through the beryllium copperto the inner core portion 12. This, combined with the increased surfacecontact as a result of undulating surface 22 of the inner core portion12, provides a more even temperature distribution along the runnerpassage 16 as well as avoids hot spots along the heating element 26which are otherwise inefficient and may result in the element burningout. In order to avoid release of the seal during filling, the brazingpaste should be selected to have a melting temperature at least about50° F. above that of the beryllium copper alloy. Heat treating may alsobe carried out during this filling step if it has not been done duringthe sealing step or previously. This requires that the heating times andtemperatures are appropriate to the assembly materials being treated, aswell as to carry out filling without releasing the seal. As illustratedin FIG. 3, assemblies 10 are filled in the vacuum furnace 60 in batchesin order to improve the efficiency of manufacture.

After the filled assembly cools, it is finished merely by removing theplug plate 62 and machining to remove the projecting portion of theouter sleeve 40. As seen in FIG. 4, this forms an integral sprue bushing70 with a highly conductive portion 72 cast over the heating element 26between the corrosion resistant core portion 12 and outer sleeve portion74. The conductive portion 72 also extends into a further sleeve portion76 to encase the leads 28 which extend therethrough. Not only is costlymachining of the outer surface of the sprue bushing eliminated, theamount of beryllium copper alloy required is reduced and very little iswasted. In particularly corrosive applications, the inner core portion12 may be formed of a beryllium nickel alloy, a chromium nickel steel ora chromium nickel alloy such as Inconel. It must have sufficientstrength to withstand the repeated high pressure loading, but theintegral structure with the outer sleeve portion 74 provides additionalstrength which allows the thickness of the inner core portion 12 to bereduced which, in turn, improves the heat conductivity from the heatingelement 26 to the melt.

In use, this particular sprue bushing 70 is used for edge gate moldingand is installed in a cavity plate to extend between a molding machineand a number of cavities. The cold terminals 28 are connected to a powersource and, after the sprue bushing has heated up to operatingtemperature, operation commences. Melt from the molding machine isinjected under very high pressure into the runner passage 16 whichconveys it in a molten state to the channels 18 through which it passesto the respective gates and into the cavities. After the cavities arefilled, the melt pressure released to provide for ejection of thesolidified products and the process is repeated according to apredetermined cycle. The even provision of the minimum sufficient degreeof heat along the runner passage is important to the system performingreliably for long periods without malfunction.

Referring now to FIGS. 5 and 6, they illustrate assembly for forming thesame sprue bushing by a method of filling in the opposite directionaccording to a second embodiment of the invention. As most of thefeatures are identical to those described in regard to the methodaccording to the first embodiment, common features are described andillustrated using the same reference numerals. This assembly 10 also hasan inner core portion 12 with an undulating outer surface 22 which isintegral with an enlarged front portion 14. A runner passage 16 extendscentrally through the inner core portion 12 and connects with a numberof radial channels 18 in the front face of the front portion 14. Ahelical electric heating element 26 is slipped over the inner coreportion 12 with its coils just clearing the outer surface 22 of theinner portion 12. As mentioned above, the helical ridge 24 of the outersurface 22 is threaded in the opposite direction to the coils of theheating element 26 so that there is a minimum of contact between them.The front portion 14 has the channels 18 which extend radially outwardacross its front face from the runner passage 16 and also wells 20 whichextend between the channels nearby to the front face. The heatingelement cold terminals 28 are inserted through a circular aperture 38 ina back collar portion 32 having a cylindrical seat which is then fittedover the end of the inner core portion 12. The smaller sleeve 42 and thelarger outer sleeve 40 are then added in the manner described above. Thesmaller sleeve 42 has a cuff 52 with a hole 54 through which the stem 30of the heating element extends. One end of the outer sleeve 40 isreceived in the back collar portion 32, while the outer end forms apress fit over the front 14 to enclose a space 56 around the helicalheating element 26.

However, as may be seen, according to this method the assembly 10 isoriented in the opposite direction. The back collar portion 32 has aback wall 78 with at least one filling hole 80 therethrough and a collar82 is fitted over the back collar portion 32 to form an upwardly openmouth 84. After the assembly 10 is sealed against leakage according toone of the steps described above, a slug 66 containing a predeterminedquantity of highly conductive material is placed in the upwardly openmouth 84 and the assembly is heated in the vacuum furnace 60 to melt it.The material runs down through the filling hole 80 to fill the space 56around the heating element 26 as well as the further space 58 around theleads 28 and the wells 20 in the front portion 14. As described above,filling under a partial vacuum causes the beryllium copper to form animproved contact with the adjacent materials. This increases theeffectiveness of heat transfer by the conductive portion 72. This,combined with the added strength provided by the stainless steel outersleeve portion which allows the corrosion resistant inner portion 12 tobe thinner, reduces fluctuations in temperature along the runner passage16 and temperature build-ups adjacent the coils of the heating element26.

After filling, the assemblies are removed from the vacuum furnace andallowed to cool. The collar 82 is removed to provide a sprue bushingwith little or no finishing required. It is, of course, apparent thatother configurations of the front portion 14 could be used to providefor other types of gating.

FIG. 7 illustrates another method of filling the assembly according tothe invention. In this embodiment, the assembly steps and components arethe same and need not be described further, except that a funnel 86 isinserted into a hole 88 in the outer sleeve 40. The slug 66 of highlyconductive material is inserted into the funnel 86 and the assembly isthen inserted into the vacuum furnace 60 where it melts and flows downand in between inner core portion 12 and outer sleeve 42. After cooling,the funnel 86 and the extruding portion of the conductive portion 72 aremachined off to provide the sprue bushing with a smooth outer finish. Inthis embodiment, the front portion 14 is provided with a generallyconical configuration to provide for straight center gate molding.

FIG. 8 shows a sprue bushing assembly according to another embodiment ofthe invention wherein the assembly components and steps are the same asthose described in the foregoing except that a thermocouple 90 with alead 92 is provided to monitor the temperature near the gate area. Asmay be seen, the thermocouple lead 92 extends through the smaller sleeve42 beside the heater element stem 30, through the space 56 between theinner core portion 12 and the outer sleeve portion 74, and into one ofthe wells 20 where the thermocouple is positioned near the adjacentchannels 18. When the assembly is filled by one of the methodsdescribed, the thermocouple 90 is covered with the highly conductivematerial. This protects the thermocouple and the lead and thethermocouple provides an indication of the degree of heat the materialis conducting to the area near the gate.

The sprue bushing 70 shown in FIG. 9 is similar to that shown in thefirst embodiment except that the inner core portion 12 has a cylindricalouter surface 98 rather than an undulating outer surface. When the spruebushing is formed according to the method described herein, the highlyconductive portion 72 is fused to the cylindrical outer surface 98,providing sufficient heat transfer therebetween. Otherwise, the methodof manufacture and use is the same as that described above and need notbe repeated.

While the description of the sprue bushing and methods of making it havebeen provided with respect to several embodiments, it is not to beconstrued in a limiting sense. Many variations and modifications may nowoccur to those skilled in the art. In particular, componentconfigurations and materials may be used other than those describedprovided they have the necessary characteristics. Surfaces may bepainted with titanium oxide paint to avoid fusion to the highlyconductive filling material referring to the embodiment shown in FIGS. 1and 2, titanium oxide paint may be applied, for instance, to the frontface of the front portion 14 rather than using plug plate 62. Then afterthe bushing has cooled, the conductive material may be scraped orbrushed off to uncover the radial channels 18. Reference is made to theappended claims for a definition of the invention.

What I claim is:
 1. An integral electrically heated sprue bushingcomprising:(a) a hollow elongated inner core portion extending betweenan enlarged front portion and an enlarged back collar portion, defininga central runner passage extending therethrough, the core portion, theenlarged front portion and the enlarged back collar portion being formedof a corrosion resistant material and having an outer surface; (b) anelectrically insulated helical heating element having a plurality ofspaced coils encircling the inner core portion and lead wires extendingthrough an aperture in the back collar portion, the inner helicaldiameter of the coil being greater than the maximum outer diameter ofthe outer surface of the inner core portion; (c) an elongated outersleeve portion formed of a corrosion resistant material extendingbetween the front portion and the back portion to enclose a first spacearound the heating element between the front portion and the backportion; (d) a highly thermally conductive metal portion cast into saidspace between the inner core portion and the outer sleeve portion tocast in the coils of the heating element; and (e) a further sleeveportion with one end received in the aperture in the back collar portionto receive the lead wires extending therethrough, said further sleeveportion also being filled with highly conductive material to encase theportion of the lead wires extending therethrough.
 2. A sprue bushing asclaimed in claim 1 wherein the front portion has a front surface with aplurality of channels therein extending radially outward from thecentral runner passage extending therethrough, whereby in use each ofthe channels conducts melt to a corresponding edge gate.
 3. A spruebushing as claimed in claim 1 wherein the front portion has a pluralityof wells therein extending from the said first space, the wells alsobeing filled with said highly conductive material.
 4. A sprue bushing asclaimed in claim 3 wherein a thermocouple is positioned in one of thewells to monitor the temperature near the gate.
 5. A sprue bushing asclaimed in claim 1 wherein the outer surface of the inner core portionis undulating.
 6. A sprue bushing as claimed in claim 1 wherein theouter surface of the inner core portion is cylindrical.